1. Field of the Invention
This invention relates to a phenolic resin composition useful for preparing reinforced composites. The present invention particularly relates to a resole-type phenolic resin prepared using an ortho directing catalyst and containing a borate salt which is useful for preparing reinforced composites. The resole resins of the present invention possess superior water dilutability and storage stability, reduced volatile emissions upon curing, and excellent compressive strengths, and flexural strength when used to make reinforced composites. The reinforced composites may use inorganic and/or organic fibers, which may be chopped, non-woven or woven, and honeycombed materials as the reinforcing materials, e.g. to form prepregs which are laminated and cured, and cured honeycomb composites for various conventional applications, particularly those requiring excellent flame and smoke properties.
2. Description of the Prior Art
Phenolic resins have long been employed as binders for use with glass fibers in the preparation of fiber reinforced composites. Glass fibers which may be chopped, non-woven or woven are coated or saturated with the aqueous binder solution, usually by dipping or spraying, and passed through an oven where they are compressed to the desired thickness and density, and then permanently fixed by heat setting or curing the resin binder. Phenolics are preferred over many other polymer systems, such as polyesters and epoxies, due to their well known excellent resistance to flame and smoke generation. Thus, their use in the aerospace, mass transit, and other applications requiring excellent flame and smoke properties is expected to grow. However, environmental considerations have directed the composite industry away from the solvent-based phenolic resins. Efforts to employ water-based phenolics have previously resulted in problems with reduced strengths and poor water dilutability during storage.
The properties desired of binder compositions depend to a large extent on the properties of the basic resin. A good binder composition should above all be easily applied and capable of covering and/or bonding the reinforcing components, e.g. fibers and honeycombs, and at the same time it should cause little pollution. Further, the resin should have good long term stability and a high degree of dilutability with water. Since the concept of dilutability is particularly important, it will be defined for the purposes of the present invention as follows: The water dilutability of a resin solution is the volume of de-ionized water which can be added at a given temperature to one unit volume of this solution without producing any permanent perturbation, i.e., haze, clouding or precipitation.
Of particular interest are high efficiency single phase aqueous phenolformaldehyde resins which have low free phenol and low free formaldehyde. Such resins retain a high percentage of the organic moiety when the resin is cured. However, free phenol and free formaldehyde volatilize in the curing of the resin causing pollution considerations and also reducing the efficiency and performance of the resin in various bonding applications. Accordingly, it is necessary that the resin should be as free as possible from any unconverted starting materials or residues thereof while preserving its useful qualities. The risk of atmospheric pollution is in fact mainly due to the presence of volatile monomers. These consist, for example, the starting materials required for producing the resin, e.g. formaldehyde and phenol, which have been left unconverted by the reaction or regenerated in the course of binding the fibers or subsequently.
The utilization of boron compounds in hardenable polymeric binder materials, and in particular the binder which are based on phenol-formaldehyde condensation products is known. It is also generally known that such boron compounds are useful in imparting flame retarding properties to such condensation products. See U.S. Pat. Nos. 2,941,904, 2,990,307 and 3,218,279 to Stalego and U.S. Pat. No. 4,176,105 to Miedaner.
U.S. Pat. No. 4,480,068 to Santos et al. discloses that prior art attempts to employ borates in sufficient quantities needed to give improved thermal resistance have frequently resulted in resins which exhibit poor storage stability and poor tensile strength. The reason for this seems to be that the addition of larger amounts of boric acid disrupts the cure properties of the binder and, thus, affects the final strength of the bonds which are formed. In Santos, these disadvantages are overcome by using a special binder. The binder is prepared in an aqueous medium by mixing (1) a mixture of boric acid and a soluble hydroxyl compound, (2) a nitrogen containing compound, and (3) a phenol-formaldehyde resole modified with urea, then (4) adjusting the pH to 8.5-9.5.
According to U.S. Pat. No. 2,748,101 to Shappell, boric acid may be used to improve aging characteristics and water dilutability of an alkali metal catalyzed phenol-formaldehyde resin.
The prior art also discloses the use of boron containing compounds as catalysts or accelerator for phenol-aldehyde resins. See U.S. Pat. No. 2,606,888 to Williams et al. (novolac); U.S. Pat. No. 2,864,782 to Mitchell (phenolic, melamine and urea resins); U.S. Pat. No. 3,839,236 to Foley et al. (tertiary condensation product of phenol, formaldehyde and a silane); U.S. Pat. No. 4,584,329 to Gardziella et al. (phenol-aldehyde); and British Pat. No. 1,055,637 (phenolic resin). In U.S. Pat. No. 3,332,911 to Huck, boric acids or borate salts of divalent electropositive metals which are above hydrogen in the electromotive series are used as catalysts in both novolac preparation and curing stages.
The prior art also discloses the use of boron containing compounds as emulsifiers or viscosity enhancers. See U.S. Pat. No. 1,999,715 to Billings et al. (borax as an emulsifying agent); U.S. Pat. No. 2,889,241 to Gregory et al. (boric acid as a thickener); and U.S. Pat. No. 4,824,896 to Clarke et al. (borax as a thickener and to improve bonding at lower press time). In U.S. Pat. No. 4,123,414 to Milette, boric anhydride is used as a hydrophilous inorganic compound. However, to avoid the viscosity increase associated by adding the boric anhydride to the resole beforehand, the method thereof involves simultaneously projecting into a mold a resole resin, glass reinforcing elements, an acid catalyst and boric anhydride.
Various references directly or inferentially teach away from the use of electropositive divalent metals as condensation catalysts in the preparation of phenolaldehyde resins if boron containing compounds are to be included therein. For example, U.S. Pat. No. 2,748,101 to Shappell discloses suitable condensation catalysts are those that do not form water-insoluble borate salts or complexes upon the addition of boric acid thereto. Barium hydroxide and calcium hydroxide are disclosed as unsatisfactory catalysts, in that they yield insoluble products with boric acid. Alkali catalyzed phenol-formaldehyde resins are also used in U.S. Pat. No. 4,480,068 to Santos et al. which also adds boric acid. U.S. Pat. No. 4,985,489 to Barker et al. discloses that catalysts such as zinc acetate are less desirable because they yield resins having a mixed bridging structure containing ortho-ortho benzylic ether bridges and ortho-para methylene bridges which reduces the desired capacity of the resin for complexing with the oxyanions, e.g. borate ions. Molecules in which the phenolic residues are linked by ortho-ortho methylene bridges have very few sites for complexing with oxyanions, and it is therefore desirable that such molecules be absent, or if present, present in relatively small numbers. The oxyanions form a stable complex with the resin when carbon dioxide gas is passed through the formed articles containing the resin and oxyanions, thereby curing the resin. When zinc acetate was used, the properties, e.g. compression strength, of the binders were inferior to those of similar binders obtained using alkali catalyzed resole phenol-formaldehyde resins due to the reduced capacity in the zinc acetate catalyzed resins for complexing with borate ions.
U.S. Pat. No. 5,032,431 to Conner et al. discloses a phenolic resole composition useful in a glass fiber insulation binder. The binder is an aqueous mixture of a phenolformaldehyde resole, a water-soluble-borate and optionally a silane coupling agent. The resole is prepared using alkali metal hydroxides, alkali metal carbonates and tertiary amines as suitable condensation catalysts. Such catalysts promote para-para and ortho-para condensation and discourage ortho-ortho condensation. However, like the prior art, this system suffers from increases in viscosity when increasing amounts of the water-soluble borate is added thereto.
The present invention is based on the discovery that the addition of a proper amount of a water-soluble borate compounds to a phenolic resole resin in which the aldehyde groups are predominately substituted ortho to the phenolic hydroxyls (forming ortho methylol phenols) results in products with unexpectedly increased water dilutability even at pH values in the range of 7 to 8.5. Such phenolic resins will also accommodate higher borate contents without the large viscosity increases observed with the practice of the prior art, particularly Conner et al. Dramatic reductions in volatile emissions during cure were also obtained as well as large increases in compressive strengths and flexural strength measured on glass fiber reinforced laminates prepared using the resins of the present invention.